Applications sometimes need to establish and manage a session between computing devices, referred to as endpoints. A session is a set of interactions between computing devices that occurs over a period of time. As an example, real-time communications applications such as MICROSOFT WINDOWS MESSENGER or Voice over Internet Protocol (“VoIP”) establish sessions between communicating devices on behalf of a user. These applications may use various mechanisms to establish sessions, such as a “Session Initiation Protocol” (“SIP”). SIP is an application-layer control protocol that devices can use to discover one another and to establish, modify, and terminate sessions between devices. SIP is an Internet proposed standard. Its specification, “RFC 3261,” is available at <http://www.ietf.org/rfc/rfc3261.txt>.
A SIP network comprises entities that can participate in a dialog as a client, server, or both. SIP supports four types of entities: user agent, proxy server, redirect server, and registrar. User agents initiate and terminate sessions by exchanging messages with other SIP entities. A user agent can be a user agent client, which is generally a device that initiates SIP requests, or a user agent server, which is a device that generally receives SIP requests and responds to such requests. As examples, “IP-telephones,” personal digital assistants, and any other type of computing device may be user agents. A device can be a user agent client in one dialog and a user agent server in another, or may change roles during the dialog. A proxy server is an entity that acts as a server to clients and a client to servers. In so doing, proxy servers intercept, interpret, or forward messages between clients and servers. A redirect server accepts a SIP request and generates a response directing the client that sent the request to contact an alternate network resource. A registrar is a server that accepts registration information from SIP clients and informs a location service of the received registration information.
SIP supports two message types: requests, which are sent from a client to a server, and responses, which are sent from a server to a client, generally when responding to a request. A SIP message is composed of three parts. The first part of a SIP message is a “start line,” which includes fields to indicate a message type and a protocol version. The second part of a SIP message comprises header fields whose values are represented as name-value pairs. The third part of a SIP message is the message's body, which is used to describe the session to be initiated or contain data that relates to the session. Message bodies may appear in requests or responses.
SIP messages are routed based on the contents of their header fields. To be valid, a SIP request should contain at least the following six header fields: To, From, CSeq, Call-ID, Max-Forwards, and Via. The To header field indicates the logical identity of the recipient of the request. The From header field indicates the logical identity of the initiator of the request. The Max-Forwards header field indicates the number of hops a request can make before arriving at its destination. As an example, if a message from device A transits device B before arriving at destination device C, the message is said to have made two hops (e.g., to devices B and C). The Via header field indicates the path taken by the request so far (e.g., a sequence of network addresses of devices through which the request has transited) and indicates the path that should be followed when routing the response. Various network devices may insert Record-Route header fields when forwarding a SIP message in an attempt to force subsequent messages in a dialog to be routed through the device. The Record-Route header field may contain an identifier (e.g., network address) for the device and parameters. Devices that handle a message may force the message to be routed to devices listed in a message's Route header field. The Route header field values may be based on the Record-Route header field values inserted by devices. These and other header fields are described in the SIP specification referenced above.
A common form of real-time conversation is provided by instant messaging services. An instant messaging service allows participants at endpoints to send messages and have them received within a second or two by the other participants in the conversation. The receiving participants can then send responsive messages to the other participants in a similar manner.
When a participant at an endpoint wants to establish a multiparty session, for example, for sending instant messages between participants, the endpoints may be connected in a mesh configuration to support the signaling needed for the session. In a mesh configuration, each endpoint has a connection to each other endpoint. In the SIP protocol, an endpoint is connected to another endpoint when a dialog is established between the endpoints. To establish a connection with each other endpoint, each endpoint needs to be aware of the other endpoints in the session. Each endpoint that is to be in a multiparty session implements a serial invitation protocol for coordinating the sending of invitations and acceptances to establish the dialog between the endpoints in the session. A serial invitation protocol is used because, when an endpoint is invited, that endpoint needs to know about all the other endpoints currently in the session so that it can establish a dialog with each of those other endpoints. As a result, the endpoint that initiates the session needs to wait until it receives an acceptance or rejection from each endpoint before inviting another endpoint. The delay resulting from sending out the invitations serially is typically acceptable because an endpoint implementing the serial invitation protocol can automatically accept an invitation to establish a dialog of a session without having to ask the participant's permission to accept the invitation. Because each endpoint automatically accepts the invitation, there is typically very little delay between the sending of the first invitation and the receiving of the acceptance of the last invitation.
A difficulty occurs, however, when an endpoint no longer automatically accepts an invitation, but rather seeks permission from the participant to accept the invitation or delays for some other reason before automatically accepting an invitation. In such a case, there can be a considerable delay between when the first invitation is sent and when the last acceptance is received because of the aggregate delays of the endpoints. Moreover, in some cases, if an endpoint never responds to an invitation, the delay can be indefinite.